1. Field of the Invention
This invention relates to electronic circuits used to control voice coil motors (VCM's) used in disk drives and more particularly to feedback circuits used to reduce the distortion in the acceleration profile of the voice coil motor actuated heads in a disk drive.
2. Description of the Relevant Art
The problem addressed by this invention is encountered in the head positioning systems in the disk drive industry. Typically, a modern fixed disk drive has two or more double sided disks and over a thousand tracks per disk. Each track is divided into sectors. Each side of a disk requires at least one head to read and write information onto the surface of the disk. The information is usually grouped by surface, track, and sector. Consequently, an important performance characteristic of a disk drive is how quickly the heads of a disk drive can move from one track to another, commonly referred to in the industry as head seek time. The faster the head seek time, the higher the performance of the disk drive since the transfer rate of the information is increased as head seek time is decreased.
It has become common in the disk drive industry to use voice coil motors (VCM's) to move the heads in a disk drive. Voice coil motors offer the advantages of higher speed and higher track to track resolution than stepper motors.
FIG. 1 shows a typical circuit for controlling a voice coil motor 10 as is known in the prior art. The circuit is commonly referred to as an H-configuration because the four n-channel transistors 12, 14, 16, and 18 form an "H" around voice coil motor 10. It is understood in the industry that the transistors in the H-configuration can be any common transistor such as bipolar transistors and the like. In operation, controller 20 controls the position of the heads of a disk drive by controlling the current in the voice coil motor 10. For example, to accelerate the heads in a first direction, the controller would send an enabling signal to transistor 22 turning on transistor 12 and thus applying a high voltage to node 30. At the same time, controller 20 sends a DAC signal to amplifier 34 to turn on transistors 28 and 18 proportional to the strength of the DAC signal, pulling node 32 low. Since node 30 is at a higher voltage potential than node 32, current flows from node 30 to node 32 and the heads of a disk drive will respond to the resulting magnetic field by accelerating proportionally to the current magnitude. Conversely, the heads of a disk drive are accelerated in the opposite direction by enabling transistor 26 and transistor 14, thereby raising node 32 to a higher voltage potential than node 30. The resulting current flow will accelerate the heads in the opposite direction.
To achieve a fast head seek time, the head is conventionally accelerated until it is half way to the desired track, and then decelerated until the head reaches its destination, as shown in FIG. 2. This acceleration-deceleration profile is ideally accomplished by driving current through the voice coil to accelerate the heads and then reversing the current through the voice coil to decelerate the heads. However, during acceleration, heads store energy in the form of kinetic momentum and back electromotive force (BEMF). If the prior art H-bridge circuit is used, the BEMF will drive the voltage on node 30 to a voltage equal to Vcc plus the BEMF which can exceed the safe operating limits of the circuit elements. To protect the circuit elements, the BEMF voltage is typically clamped by diodes, such as diode 13, to a safe operating voltage. When the BEMF voltage is clamped, it distorts the acceleration profile as shown in FIG. 3. This distortion causes excessive wear and drive noise which negatively affects the disk drive.
Therefore, it is an object of the invention to eliminate the distortion in the acceleration profile due to the clamping of the BEMF voltage.
It is further an object of this invention to eliminate the diodes used to clamp the BEMF voltage.
These and other objects, features, and advantages will be apparent to those skilled in the art from the following detailed description when read in conjunction with the accompanying drawings and appended claims.